1. Field of the Invention
The invention relates generally to the field of digital content distribution. In one exemplary aspect, the invention relates to methods and apparatus to create, transmit, receive and decode wideband transport streams comprising one or more service levels, such as standard definition (SD) and high definition (HD) television programs.
2. Description of Related Technology
In conventional cable television networks, the available spectrum for transmission to customer premises (“downstream spectrum”) is divided into RF channels that have a given frequency bandwidth (e.g., 6 MHz in the United States) and occupy spectral slots between a higher and lower bound, such as 54 MHz and 860 MHz. A 6 MHz wide channel is used to carry, for example, a single channel analog television program or a multi-program digital television stream (broadcast or on-demand) or a DOCSIS downstream transport stream to the customer premises.
Since cable signals are transmitted using a Quadrature Amplitude Modulation (QAM) scheme, available payload bitrate for typical modulation rates (QAM-256, QAM-64) used on HFC systems is roughly, e.g., 38 Mbps (for QAM-256). For conventional cable networks that use MPEG-2 audio/video compression technology, the available bandwidth (38 Mbps) on one 6-MHz channel can accommodate a group of approximately 10-12 standard definition television programs. This group of television programs is typically created at a cable headend by using a program multiplexer. FIG. 1 shows functional block diagram of such a multiplexing stage typically used at a headend. In this architecture, a pre-determined number of programs in MPEG-2 format comprise a series of inputs 100 to a quantization stage 102. The quantization stage is so called because it has the ability to alter the bitrate of each stream in real time. These rate-altered programs are multiplexed together in a multiplexing stage 104 that produces an output transport stream 106 with the target rate suitable for modulation over a single channel carrier. The multiplexing stage also provides substantially real time feedback to the quantization stage via feedback loop 108.
High Definition Services
In recent times, High Definition television (HDTV) has become an increasingly popular format for transmission and viewing television programming. More and more households and other premises are beginning to purchase High Definition (HD) televisions (HDTV). To keep up with this trend, cable operators have begun offering HD television programs to cable customers, and have recently also started deploying HD “on-demand” (OD) services.
Transmission of an entertainment-quality HD program requires about 4-to-6 times as much bandwidth as an SD program. For example, terrestrial HDTV signals are broadcast using 19.2 Mbps bitrate for video, which is substantially more than the typical 3-4 Mbps rate used for transmission of entertainment-quality SD program over cable networks. In spite of the use of latest video compression techniques, cable operators still require upwards of 12-15 Mbps bitrates for transmission of MPEG-2 Main Profile—High Level (MP@HL) HD programs.
Since transmission of HD programs results in a significantly higher usage of network bandwidth, network operators are looking for improved methods to reduce this burden by creating bitrate-efficient multiplexes of HD programs.
Statistical Multiplexing of HD Signals
Statistical multiplexing is a technique used to efficiently pack multiple programs within a transport stream. This technique relies on the principle that instantaneous bandwidth required to transmit a program fluctuates over time, typically based on the ease of compression of the video content. This makes bandwidth-efficient transmission of multiple programs possible as a multiplex by staggering peak bandwidth requirements of the programs. Conventional approaches to statistical multiplexing have recognized that the greater the number of programs in a multiplex (i.e., “pool size”), the better the chances of using bandwidth efficiently.
However, in the context of transmission of HD programs over cable networks using conventional techniques, the applicability of statistical multiplexing is limited. The main reason for this is that due to the relatively high bitrates of HD programs, a typical transport stream corresponding to a 6 MHz cable channel can only carry 2 to 3 HD programs. This limits the effectiveness of statistically multiplexing HD programs in a multiplex with other HD or SD programs.
Accordingly, a technique that allows for the creation and transmission of transport multiplexes that comprise a larger number of HD programs, thereby improving the opportunity to efficiently group the programs for transmission over networks, will be useful to address the bandwidth burden problem. Such a transport multiplex will typically have a bitrate higher than what can be sent on one RF channel; hence the term “wideband multiplex” can be used to refer to such a multiplex.
U.S. Pat. No. 5,708,664 to Budge, et al. issued Jan. 13, 1998 entitled “Statistical multiplexing” discloses a transmitter for transmitting a plurality of digital signals through a plurality of channels, the channels having a predetermined total allocated bitrate. The transmitter includes a plurality of encoders each associated with one channel, a multiplexer for receiving the encoded digital signals and for transmitting the encoded signals as a stream of data, and operable for adjusting the distribution of the bitrate allocation between and among the encoded signals, and a processing device for providing an indication of a target quality and an actual quality for each channel and for causing the multiplexer to repeatedly adjust the distribution of the bitrate allocation in response to differences between the indicated actual quality and the indicated target quality for each channel so as to equalize differences between the actual and target quality across at least some of the channels. By grouping encoders together in “statistical multiplex groups”, and making real time decisions about the bitrate requirements for those encoders, bitrate can be allocated to maximize picture quality for the group. For a variety of different picture sources in a statistical multiplex group, to achieve a target picture quality the bitrate requirements of each will vary with coding difficulty. Thus, a channel within the statistical multiplex group that is experiencing little difficulty in encoding its picture can free bits to channels that are having greater difficulty. The effect is to smooth the picture quality and subjectively improve it.
U.S. Pat. No. 6,219,358 to Pinder, et al. issued Apr. 17, 2001 entitled “Adaptive rate control for insertion of data into arbitrary bit rate data streams” discloses apparatus wherein the rate of insertion of data, such as MPEG table packets, into an outgoing bit stream is varied by a packet handler. The packet handler, which is located in a modulator in a cable television system headend, comprises control logic and a packet router. The actual insertion rate of the outgoing data is based on the bit stream's available capacity for insertion of data and the desired insertion rate of the data. When the available capacity for insertion equals or exceeds the desired insertion rate, the actual insertion rate equals the desired insertion rate. When the available capacity for insertion is less than the desired insertion rate, the actual insertion rate is reduced from the desired insertion rate. The invention dynamically determines the available capacity for insertion and adjusts the actual insertion rate.
United States Patent Publication 20010055336 to Krause, et al. published Dec. 27, 2001 and entitled “Compressed-Video Reencoder System For Modifying The Compression Ratio Of Digitally Encoded Video Programs” discloses a compressed video decoder/encoder (reencoder) system for varying the compression ratio of a compressed video program. The composite reencoder system implements tightly coupled elements for decoding and encoding compressed video data implementing techniques of header forwarding and utilizing an architecture in which a shared motion compensator supports both decoding and encoding operations simultaneously. The reencoder system may be introduced in a statistical multiplexer for generating a compressed video data stream multiplex suitable for use in cable distribution and other video distribution systems.
United States Patent Publication No. 20020085584 to Itawaki, et al. published Jul. 4, 2002 entitled “Statistical multiplex system, statistical multiplex controller and method of statistical multiplex” discloses a statistical multiplex system, a statistical multiplex controller and a method of statistical multiplex, which can assign bit rates to program data and auxiliary data, ostensibly to improve image quality. A statistical multiplex system is provided with: a plurality of image encoders for encoding a plurality of program data; an information encoder for encoding the auxiliary data; a multiplexing apparatus for multiplexing output thereof, and a statistical multiplex controller for controlling each of the image encoders and the information encoder. The statistical multiplex controller is made to set the bit rate to be assigned to the information encoder first, and to assign the remaining bit rates to each of the image encoders.
United States Patent Publication No. 20030083054 to Francesca, et al. published May 1, 2003 and entitled “Multi-channel broadband content distribution system” discloses a system for managing bandwidth in a content distribution system. The system can be incorporated into the content head end of the content distribution system. The system includes a program multiplexer, a multi-channel modulating module, a channel multiplexer, a digital-to-analog converter and a frequency block-up converter, all arranged in a sequential configuration. Packets representing respective content programs are fed to the program multiplexer. The program multiplexer multiplexes the packets into an output queue. How the packets are multiplexed by the program multiplexer into the output queue depends on the specific design and/or application. Packets from the output queue are then fed to the multi-channel modulating module. The multi-channel modulating module receives the packets and routes them to various modulators representing corresponding RF channels. The various modulators then modulate the respective packets to generate corresponding RF signals. These RF signals are then multiplexed by the channel multiplexer into a multi-channel RF signal. The multi-channel RF signal is then forwarded to the digital-to-analog converter for conversion into an analog, multi-channel RF signal. The frequency block-up converter then takes the analog multi-channel RF signal and shifts its to a higher frequency band for transmission. The shifted analog multi-channel RF signal is then transmitted over a medium to one or more customer premises equipment. Notwithstanding the foregoing, this solution fails to provide mechanisms for performing the important steps of encryption at a headend, and corresponding decryption at the customer premises equipment (CPE). Also, a corresponding CPE configuration that can receive wideband signals (essential to the operation of such a system) is not provided.
Based on the foregoing, it is evident that while the prior art has in general recognized the utility of wideband content transmission, it lacks apparatus and methods to adequately implement this approach. Specifically, prior art does not teach how to create transport multiplexes of the type needed to efficiently and effectively transport such wideband signals by enforcing various multiplexing rules across multiple carriers, nor does it address backwards compatibility with legacy devices.
What is needed are improved apparatus and methods to create, transmit and receive wideband multiplexes that allow flexible encryption and multiplexing of programs and program information tables, and which provide backwards compatibility with legacy customer premises equipment. Such improved apparatus and methods would also, inter alia, allow for a larger statistical multiplex pool size, thereby increasing the leverage in efficiency provided by the statistical multiplexing approach in an HD environment.